Work resulting from this project revealed that yersiniae fail to produce detectable phenolate or hydroxamate siderophores in medium containing less than 0.3 muk M Fe 3 ion but do possess a low affinity cell-bound Fe 3 ion-transport system and can utilize hemin as a sole source of iron. Proposed work will show, by use of mutants blocked in the Fe 3 ion or hemin transport systems, that hemin is a favored source of iron in vivo. An attempt will be made to show that virulence of mutants blocked in Fe 3 ion uptake can be phenotypically enhanced by small (non-utilizable) quantities of injected Fe 3 ion. This result would demonstrate that infected iron inhibits mechanisms of host defense. The kinetics of Fe 3 ion and hemin uptake will be determined with respect to Km and inhibitors of proton-motive force and ATP generation. The nature of the hemin binding site, characterized as an envelope protein unique to pigmented yersiniae, will be determined using the bacteriocin pesticin as a probe. The consequences of the pleiotrophic Wyb defect in wild type Yersinia pestis but not Y. pseudotuberculosis will be further characterized with respect to porphyrin synthesis and oxidative amino acid catabolism.